Our studies will focus on baculovirus envelope proteins as models for viral envelope protein function and interaction with host cells, especially the polarized epithelial cells of the insect midgut. Our efforts will focus on four specific areas: 1) Viral receptor binding; 2) Envelope protein targeting in insect polarized midgut epithelial cells; 3) Virion assembly and budding; and 4) Physical structure of the GP64 protein. Studies of viral receptor binding will focus on the GP64 protein. We will first identify the receptor-binding domain of the GP64 protein and we will then identify the host cell receptor. Because baculoviruses, like many viruses transmitted by insects, must move infectious virus particles through the midgut barrier, we will examine GP64 as a model for studies of viral movement through the insect midgut epithelium. GP64 is targeted to the basal membranes in polarized midgut epithelial cells, where virions bud into the hemocoel. Our studies will identify the midgut-targeting domain of GP64 and we will ask whether GP64 targeting directs virion budding from basal surfaces of the midgut epithelial cells. Recombinant viruses containing modified forms of the GP64 protein will be used to map the basal targeting domain and we will examine interactions of GP64 and other viral or cellular proteins in this process. Because GP64 is critical for efficient virion budding, we will also use similar techniques to identify and characterize the GP64 budding domain and viral or cellular proteins that interact with GP64 during assembly and budding. GP64 appears to represent a structurally and functionally unique class of viral envelope fusion protein. To accelerate our understanding of GP64 structure-function relationships, we will devote substantial efforts toward the generation of crystals suitable for structural studies of GP64. For these studies, we have developed a powerful genetic system and many tools that permit us to study GP64 in the context of budded virions and the viral infection cycle. These studies will advance our understanding of viral envelope proteins and they will also have broader applications to important areas of biology, medicine, and public health.